(1) Field of the Invention
The invention relates to a process for the stabilization of and the prevention or substantial reduction of fogging in the use of photographic silver halide emulsions during the stages of their production.
(2) Description of the Prior Art
It is well-known in the art, that apart from the appearance of the silver image in developing exposed silver halide emulsion layers, there also occurs an additional darkening, called "fog." Fogging is dependent upon the properties of the emulsion and its additives as well as upon the conditions of storage and development. To prevent or to reduce fogging, stabilizers and anti-fogging agents are added to the emulsion or to the developer, usually resulting in a loss in sensitivity. An exception is the compound 5-methyl-7-hydroxy-1,3,4-triazaindolizine (hereinafter designated as "triazaindolizine"), which shows a distinct sensitizing effect aside from its good stabilizing properties. This compound, however, is not sufficiently effective against all sources of fogging. Furthermore, numerous mercapto-substituted heterocyclic compounds with good anti-fogging effects are known (e.g., 1-phenyl-5-triotetrazole). By combining these compounds with triazaindolizine, it is possible to partly compensate for their simultaneously existing tendency to desensitize. Moreover, their photographic activity may be weakened in that the mercapto group is substituted by a group which is easily cleavable. "Masked anti-fogging agents" of such kind having a sufficient stability before storing or developing usually have less of a desensitizing effect than their starting compounds. The S-acylation of mercapto compounds under formation of thiolesters may be understood as a "masking process" of such a type. S-acyl derivatives of heterocyclic mercapto compounds have already been described as anti-fogging agents (DDR Patent No. 1,173,336). However, by this latter disclosure, it is not possible to completely prevent the above-mentioned disadvantage of desensitization.